High electron mobility transistors (HEMT) having GaN (GaN-HEMT) are an example of semiconductor devices with a compound semiconductor stack structure including compound semiconductors such as nitride semiconductors. For example, high-output devices having GaN-HEMT may be used in power supply devices, and high-frequency devices having GaN-HEMT may be used in high-frequency amplifiers.
High-voltage operation of these devices results in the occurrence of current collapse, which is a phenomenon in which the on resistance is increased to lower drain current (source-drain current). The occurrence of this current collapse decreases output characteristics of the devices such as output and efficiency.
A technique to reduce current collapse is to provide an insulating film covering the surface of a compound semiconductor stack structure.
However, it has been found that when an insulating film is provided to cover the surface of a compound semiconductor stack structure as taught by the above technique, electrons are captured by traps present on the surface of the insulating film during high-voltage operation, thus causing a decrease in drain current.
That is, it has been found that the application of a high drain voltage to the above device in order to enhance output characteristics of the device produces a strong electric field which is applied to the vicinity of the gate electrode, and some of the electrons running through the channel are accelerated by this strong electric field and transit to the surface of the compound semiconductor stack structure, with the result that some of the electrons that have transited are captured by traps present on the surface of the insulating film covering the surface of the compound semiconductor stack structure, thus causing a decrease in drain current.
Thus, it has been found that while the formation of an insulating film covering the surface of a compound semiconductor stack structure as taught by the above technique may reduce current collapse compared to when there is no such an insulating film, sufficient reduction of current collapse is infeasible because traps present on the surface of the insulating film capture electrons and this causes a decrease in drain current.
The following is reference document:    [Document 1] Japanese Laid-open Patent Publication No. 2010-287605.